N-Alkylaminophenols are of extreme importance in industry as intermediates for heat-sensitive or pressure-sensitive dyes, xanthene dyes, fluorescent dyes, etc.
It is conventionally known to prepare an N-alkylaminophenol by reductive alkylation of an alkylaminophenol and an aldehyde or a ketone in the presence of an organic solvent, a catalyst for reduction, and hydrogen.
However, the conventional processes have the following disadvantage. As the conventional noble metal catalysts for use in the reductive alkylation, such as platinum or palladium catalysts, have an ability to reduce an aromatic ring when used as such, nuclear hydrogenation of the aromatic ring may be caused as a side reaction depending on the reaction conditions, resulting in a decrease in the yield of the desired N-alkylaminophenols. Industrially, since these catalysts are expensive, generally it is necessary to use them repeatedly, but this has the following industrially serious problems in addition to the nuclear hydrogenation of aromatic ring. First the aminophenol and the aldehyde or ketone are condensed to form a heavy matter. Also the aldehyde or ketone is reduced to increase the by-production of an alcohol.
Of the problems, concerning the by-production of an alcohol, for example, JP-A-57-165349, JP-A-58-26844, and JP-A-58-194843 disclose techniques for suppressing the by-production of an alcohol by adding a solid sulfur compound during the course of the reductive alkylation, or using a platinum sulfide catalyst. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, though these techniques can suppress the by-production of an alcohol to a some extent, the reductive alkylation as a main reaction is also suppressed at the same time; thus there is a problem that an unstable Schiff's base of an aminophenol is condensed to form a heavy matter. These techniques also have disadvantages that when the catalyst is used repeatedly, the reaction can be controlled only with difficulty due to elimination of sulfur from the catalyst.
Further almost no conventional techniques have been mentioned about a technique for suppressing nuclear hydrogenation of an aromatic ring, especially no technique for suppressing nuclear hydrogenation of an aromatic ring in the preparation of an N-alkylaminophenol has been known.